Optimal treatment for localized prostate cancer has not been determined. While smaller tumors can be managed with what is probably equal effectiveness by either radical prostatectomy or radiation therapy, radiation therapy becomes the mainstay of treatment for larger tumors. Radiation, however, produces suboptimal clinical, long term local control in these larger tumors. In addition a significant proportion of patients who appear clinically to be locally controlled have positive post-treatment biopsies at intervals of eighteen or more months. Thus, for bulky tumors, there is both clinical and pathologic evidence that radiation therapy produces less than optimal rates of local control. Furthermore, local failure after radiation, whether clinically or pathologically detected, appear to adversely affect survival, emphasizing the need for new treatment approaches. Dose escalation, using three dimensional (3D), conformal radiotherapy treatment planning, represents one such approach and has the potential for improving local control while maintaining treatment toxicity at an acceptable level. A phase I/II study is proposed using 3D conformal radiotherapy to define a maximum tolerated radiation dose for locally advanced prostate cancer. A 3D planning system will be employed that has been developed and is in use at the University of Wisconsin. It incorporates advanced image visualization and manipulation capabilities, virtual simulation, target definition, beam and block design, 3D dose computation, dose-volume histogram analyses and portal imaging and verification protocols. Biological modeling that estimates probabilities of tumor control and normal tissue complications, with or without consideration of the impact of fraction size and dose rate, is included. The modeling assists in the evaluation of competing plans.